The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11 standard is a family of standards for wireless local area networks (WLAN) in the unlicensed 2.4 and 5 Gigahertz (GHz) bands. The current IEEE 802.11b standard defines various data rates in the 2.4 GHz band, including data rates of 1, 2, 5.5 and 11 Megabits per second (Mbps). The 802.11b standard uses direct sequence spread spectrum (DSSS) with a chip rate of 11 Megahertz (MHz), which is a serial modulation technique. The IEEE 802.11a standard defines different and higher data rates of 6, 12, 18, 24, 36 and 54 Mbps in the 5 GHz band. It is noted that systems implemented according to the 802.11a and 802.11b standards are incompatible and will not work together.
A new IEEE standard is being proposed, referred to as 802.11g (the “802.11g proposal”), which is a high data rate extension of the 802.11b standard at 2.4 GHz. It is noted that, at the present time, 802.11g is only a proposal and is not yet a completely defined standard. Several significant technical challenges are presented for the new 802.11g proposal. It is desired that the 802.11g devices be able to communicate at data rates higher than the standard 802.11b rates in the 2.4 GHz band. In some configurations, it is desired that the 802.11b and 802.11g devices be able to coexist in the same WLAN environment or wireless area without significant interference or interruption from each other, regardless of whether the 802.11b and 802.11g devices are able to communicate with each other. Thus, it is desired that 802.11g be backwards compatible with 802.11b devices. It may further be desired that the 802.11g and 802.11b devices be able to communicate with each other, such as at any of the standard 802.11b rates.
An impairment to wireless communications, including WLANs, is multi-path distortion where multiple echoes (reflections) of a signal arrive at the receiver. Both the single-carrier systems and multi-carrier systems must include equalizers that are designed to combat this distortion. The equalizer of the single-carrier system is designed based on its preamble and header. Other types of interferences, such as different and incompatible wireless signal types, may cause problems with WLAN communications. The Bluetooth standard, for example, defines a low-cost, short-range, frequency-hopping WLAN. Systems implemented according to the Bluetooth standard present a major source of interference for 802.11-based systems. Preambles are important for good receiver acquisition. Hence, losing all information when transitioning from single-carrier to multi-carrier is not desirable in the presence of multi-path distortion or other types of interference.
There are several potential problems with the signal transition, particularly with legacy equipment. The transmitter may experience analog transients (e.g., power, phase, filter delta), power amplifier back-off (e.g. power delta) and power amplifier power feedback change. The receiver may experience Automatic Gain Control (AGC) perturbation due to power change, spectral change, multi-path effects, loss of channel impulse response (CIR) (multi-path) estimate, loss of carrier phase, loss of carrier frequency, and loss of timing alignment.
A mixed waveform configuration for wireless communications has been previously disclosed in U.S. Provisional Patent Application entitled, “Wireless Communication System Configured to Communicate Using a Mixed Waveform Configuration”, Ser. No. 60/306,438 filed on Jul. 6, 2001, which is also incorporated by reference in its entirety. The system described therein reused the equalizer information obtained during acquisition of the single-carrier portion of the signal. The technique provided continuity between the single-carrier and multi-carrier segments (e.g., orthogonal frequency division multiplexing or OFDM), which was achieved by specifying the transmit waveform completely for both the single-carrier and multi-carrier segments and specifying the transition. The waveform enabled continuity between the two signal segments, including AGC (power), carrier phase, carrier frequency, timing and spectrum (multi-path). It was contemplated that the signal would not have to be reacquired by the multi-carrier portion of the receiver since the information developed during the single-carrier portion (preamble/header) was valid and used to initiate capture of the multi-carrier portion. However, particular receiver architectures were not discussed.
A mixed carrier transmitter is described herein that is capable of communicating using the proposed mixed carrier waveform configuration. The term “mixed carrier” as used herein refers a combined signal with a single-carrier portion followed by a multi-carrier portion. The transmitter may be configured to operate in multiple operating modes including single-carrier, mixed carrier and multi-carrier modes. Furthermore, several receiver architectures are described that are configured to receive a mixed carrier signal and resolve the incorporated Baseband signals incorporated in the mixed carrier signal.